In sheet-fed offset printing machines, the sheets to be printed are transported through individual printing stations and, after the last printing station, through one or more varnishing or other converting devices by means of cylinders and drums. The transport of the sheets from the last drum to the delivery stack commonly is effected by a chain-type sheet transfer system.
Drying apparatuses are provided at various locations within the printing machine in order to prevent the freshly printed sheets from becoming smeared during their transport between the individual printing stations and between the last printing station and the delivery station or the varnishing unit. The drying apparatuses are provided for drying/absorbing the ink/varnish as rapidly as possible such that any contact between the printed side of the sheet and sheet guiding plates or other stationary parts of the printing machine does not damage the printed side of the sheets and consequently, waste sheets.
Printing inks or varnishes that are based on solvents release solvent vapors during the drying process. The drying process can be accelerated with the aid of IR radiators because the evaporation of the solvent is expedited by applied heat. The drying process additionally can be accelerated if the printed sheet is subsequently impacted with warm air because the solvent vapors released by the printed sheet due to the applied heat can be transported away in this fashion. In drying processes of this type, the air that is saturated with solvent and water vapor also can be removed by suction. However, this requires a special suction apparatus.
DE 195 25 453 A1 discloses a drying method for rapidly moving webs to be printed, wherein an electrode connected to a high-voltage source is disposed adjacent the drying apparatuses. The electric charge from the electrode in the direction of the material to be printed (i.e. rapidly moving web) causes a destruction of a laminar boundary layer entrained by the moving web. This laminar boundary layer that is entrained by the web material to be printed inhibits the mass transfer of solvent and water vapor from the material being printed, i.e., the mass transfer coefficient is lowered. However, this drying principle which utilizes a high-voltage electrode can only be used on rapidly moving webs and is not suitable for sheet-fed printing machines in the form disclosed herein.